The present invention relates to a reciprocating piston drive mechanism, especially for a reciprocating piston vacuum pump, comprising a housing, a cylinder embodied in said housing, a piston moving back and forth in the cylinder and an electromagnetic drive for the piston that has an electromagnet on the stator side and at least one permanent magnet on the piston side.
A reciprocating piston drive mechanism having such characteristics is known from GB 2,241,287. In this reciprocating piston drive mechanism according to the state-of-the-art there are located in the cylinder two springs, of which one each extends between one of the face sides of the piston and the related face side of the cylinder. Through this, the piston adopts a substantially central axial position in the idle state. When continually stressing the helical springs, fatigue affecting the material of the springs is unavoidable. For this reason, the service life of reciprocating piston drive mechanisms according to the state-of-the-art is thus limited to the service life of the material employed for the springs.
The reciprocating piston drive mechanism according to GB 2,241,287 is a component of a reciprocating piston pump, in which at least one of the two chambers created by piston and cylinder has the function of a compression chamber. Located in this chamber or these chambers are the helical springs. This give rise to unwanted clearance volumes, this impairing the pumping effect.
It is the task of the present invention to improve a reciprocating piston drive mechanism of the aforementioned kind in such a manner that it no longer offers the disadvantage of spring materials being subjected to fatigue. Moreover, the design goal is such that the drive mechanism be particularly well suited for reciprocating piston vacuum pumps.
This task is solved by the present invention in that permanent magnets are provided on the stator side and where the stator magnet(s) is/are so configured and disposed that the piston adopts a substantially central axial position in the idle state. In the idle state, i.e. with the electromagnets de-energised, the superimposed magnetic fields being generated by the permanent magnets affixed to the piston and in the stator, generate forces affecting the piston holding it in a central axial position. Thus in the idle state a defined, for example, central piston position results which is solely effected by the effect of magnetic forces and does not require any additional facility of a mechanical kind, like springs.
It is expedient that the piston be equipped with two permanent magnets, of which one each is located in the area of the two face sides of the piston. Assigned to each of these permanent magnets on the side of the piston is one each permanent magnet on the stator side, specifically in the area of the face sides of the cylinder at an approximately equal radial position. In a particularly simple solution, the piston is only equipped with a permanent magnet ring arranged approximately centrally in the axial direction. Located to the side of this ring, there is located one each permanent magnet on the stator side, the distances of which with respect to the magnet ring of the piston define the amplitude of the piston""s stroke and the desired amount of piston delay as soon as it approaches one of the dead centers.
If the permanent magnets of the stator are magnetised in the axial direction with reversed polarity with respect to the corresponding permanent magnets of the piston, then their magnetic fields will generate repelling forces. These forces then have the effect that the velocity of the piston, as it approaches the face side of the cylinder, is reduced, and finally the movement of the piston in the reverse direction is initiated. If this arrangement is designed to be in all symmetrical, specifically with respect to its dimensions and also with respect to the strength of the magnetic fields, then the piston will, in the de-energised state of the electromagnet""s coil, assume a central axial position.
When employing the drive mechanism in accordance with the present invention in a reciprocating piston vacuum pump, an asymmetrical arrangement in the axial direction may be expedient, since the symmetry conditions determine the force characteristic. If the load on the two compression chambers of the pump located at the two face sides is asymmetric during the pumping process, the force characteristic can be adapted by an axially asymmetric drive mechanism.
Further advantages and details of the present invention shall be explained with reference to the schematically depicted design examples of drawing FIGS. 1 to 8.